COMPROBACIÓN DE LA HIPÓTESIS

Adapted from Noll et al. 2007, ”Presence of dust with a UV bump in massive, star-forming galaxies at 1<z<2.5”, A&A accepted

During the last year I have been involved in a project focused on the dust properties of high-redshift star forming galaxies. The main results have been published in Noll et al. (2007a,2007b).

In this study, we used a sample of 108 massive, star-forming galaxies at redshift 1 <

z < 2.5, selected from the FDF Spectroscopic Survey, the K20 and the GDDS surveys, to investigate some fundamental properties of the dust extinction curve on a galaxy SED, like the slope in the rest-frame ultraviolet (UV) and the presence/absence of a broad absorption excess centred at 2175 Å (the so–called UV bump).

The spectral features of these galaxies are parametrised by two indices: the first parameter estimates the reddening in the UV and is defined as the continuum slope measured at 1750 – 2600 Å, with the exclusion of the range 1950 – 2400 Å. This proxy for the UV reddening by dust is called βb. The second parameter characterises the apparent strength of the UV

bump and is calledγ₃₄. It is the difference between the continuum slopes measured at 1900 – 2175 Å (γ3) and 2175 – 2500 Å (γ4), respectively. A value ofγ34∼1 indicates the absence

of the 2175 Å feature. Conversely, γ34 < −2 points to an extinction law which exhibits a

significant UV bump.

These characteristics were constrained by means of a parametric description of the UV spectral energy distribution (SED) of a galaxy, as enforced by combined stellar popula- tion and radiative transfer models for different geometries, dust/stars configurations and dust properties.

In at least one third of the sample, there is a robust evidence for extinction curves with at least a moderate UV bump. The presence of particular dust particles, which are the carriers of the UV bump, is more evident in galaxies with UV SEDs suffering from heavy dust reddening.

dust grains and molecules producing extinction curves in between the average ones of the Small and Large Magellanic Cloud, where the UV bump is absent or modest, respectively. Most of the dust embeds the UV-emitting stellar populations or is distributed out of the galaxy mid-plane. Alternatively, even dust with a pronounced UV bump, as for the average Milky-Way extinction curve, can be present and distributed in the galaxy mid-plane. In this case, variations of the continuum scattering albedo with wavelength or an age-dependent extinction are not sufficient to explain the previous trend with reddening. Hence, additional extraplanar dust has to be invoked. The data suggest that the carriers of the UV bump are associated with intermediate-age stellar populations, while they survive in the harshest UV- radiation fields owing to dust self-shielding. The existence of different dust extinction curves implies that different patterns of evolution and reprocessing of dust exist at high redshift. Ignoring this may produce a non-negligible uncertainty on the star-formation rate estimated from the rest-frame UV.

6.3.0.1 Analysis of the UV morphology of high redshift galaxies

In order to investigate possible correlations between the presence of the UV bump and the UV morphology of the galaxies, I performed a morphological analysis of the sample. As I have already pointed out, at redshift larger than 1.2 even the reddest ACS band is sampling more and more the UV restrame emission of the galaxies. Thus, the morphology is more linked to the distribution of star forming regions than to the actual distribution of stars. For this reason we preferred to use a non parametric approach for this analysis.

We made use of the two indices RT and RA computed by GIM2D from the residuals

of the best fit. These indices evaluate the bumpiness and the asymmetry of the residual image, and were used to estimate the overall smoothness of a galaxy image with respect to the best fitting model. In other words, by means of these indices we can estimate the residual substructure like spiral arms in nearby late-type galaxies (see the seminal study of Elmegreen et al. Elmegreen et al. (1992)), peculiarities and/or asymmetry in the distribution of giant star-forming regions, or interaction/mergers in high-redshift galaxies (e.g., Schade et al. Schade et al. (1995)).

Since UV morphologies of high redshift galaxies are potentially much more complex than our usual parametrisation with a S´ersic profile, we also characterised the morphologies of this galaxy sample by means of two non parametric indices, namely the concentration C and the asymmetry A of the galaxy surface brightness. The concentration parameter is given by the ratio of the radius containing 80% of the total galaxy flux to the radius containing 20% of the total flux. The asymmetry parameter is obtained by rotating the galaxy image by 180 degree from its center and then subtracting it from the original image; the total flux of this residual image is then compared with the original galaxy flux.

The C-A method was developed in the mid-nineties by Abraham et al. (Abraham et al. (1994), Abraham et al. (1996)). Subsequent works (e.g., Wu Wu (1999); Conselice et al. Conselice et al. (2000), Conselice (2003); Menanteau et al. Menanteau et al. (2006)) have

Figure 6.4: S´ersic index (top) and concentration (bottom) versus the proxy for the 2175 Å

featureγ34. Black (magenta) points refer to the 1 <z<1.5 (2<z<2.5) redshift range.

Open symbols mark the subsamples with low reddening (βb < −1.5), while the highly

reddened galaxies are represented by filled symbols. Mean errors are indicated.

shown that a better morphological classification is obtained by choosing an image pivot point which minimises the measured asymmetry. We make use of the CAS parametrisation as proposed and described in detail by Conselice et al. (Conselice et al. (2000), Conselice (2003)). For our purposes in this section, it is important to say that early-type galaxies have larger concentration and lower asymmetry indices than late–type ones.

6.3.0.2 Morphological properties and extinction curve

We compared the morphological properties derived as described above for the galaxies in our sample, with the shape of their extinction curve at UV wavelengths.

We have to stress that, since the morphological parameters were determined based on the ACS i–band image of the galaxies, they correspond to different rest-frame wavelength domains, from the mid-UV to the U band, depending on the galaxy redshift.

We find that the rest-frame UV/U-band morphology of a galaxy generally does not appear

to be directly related with the shape of the extinction curve at UV wavelengths, whatever the redshift. Nevertheless, dividing our sample in “blue”/“red” objects as those havingβ_b

greater/lower than−1.5, blue objects at 2 < z < 2.5 and 1 < z < 1.5 tend to have smaller effective radii than red objects at the same redshift, whether or not a UV bump is detected in their spectra (see also Noll & Pierini (2005)).

Figure 6.5: CAS parameters concentration C and asymmetry A (Conselice et al. Conselice et al. (2000), Conselice (2003)) for our sample of FDF (lozenges) and K20 galax- ies (circles) at 1<z<1.5. Galaxies withγ34<−2 are marked by filled symbols. The solid

and dashed lines separate early-type, late-type, and merging galaxies by their asymmetry (see Conselice et al. Conselice (2003)).

have a lower nserthan red galaxies without UV bump in both redshift bins (see Fig. 6.4). For

instance, at 1<z<1.5, they exhibit an average S´ersic index of 0.54±0.25 and of 0.98±0.23, respectively.

Overall, the S´ersic index seems to be larger for galaxies at higher redshifts (Fig. 6.4). Since the available high-resolution imaging probes the rest-frame UV/U-band morphology, the

S´ersic profile describes the large-scale distribution of star-formation regions instead of the classical Hubble type. Hence, the azimuthally-averaged radial distribution of the rest-frame UV/U-band light traced by nseris shallower for galaxies with dust producing a significant UV

bump with respect to those without it, whatever the redshift. Furthermore, it is more peaked at higher redshifts than at lower ones, even though this may be an effect of the cosmological brightness dimming. In fact, a faint component of a galaxy, like a disk, can fail detection more easily at higher redshifts, thus producing a spuriously larger value of nser.

In addition to the relatively large effective radii and low S´ersic indices, the low values of concentration of the red galaxies with evidence of a UV bump at 1< z < 1.5 (see Fig. 6.4) suggest that most of these galaxies are large systems with shallow radial profiles at rest- frame UV/U-band wavelengths. This is not surprising since most of the galaxies at these

redshifts can be classified as late types (see Fig. 6.5). However, the visual inspection of the ACS images reveals that the fraction of objects with a shallow light profile in the rest-frame UV/U-band is 40 – 70% ifβ_b>−1.5, but only 20 – 40% ifβ_b<−1.5. For red galaxies with a significant UV bump, this fraction becomes larger (60 – 80%), though there are exceptions: CDFS-0271, the object with the strongest observed 2175 Å absorption feature, appears as a quite compact galaxy (Re =2.5 kpc).

Establishing the rest-frame UV morphology of a galaxy at 2<z<2.5 is not possible from the available data, owing to the increased cosmological dimming. Nevertheless, we can in- vestigate the presence of strongly distorted morphologies and/or multiple main components. These characteristics exclude the possibility of a single object with a smooth, radial surface brightness profile in the rest-frame UV. While at 1< z < 1.5 almost all galaxies show only one major component, at 2 < z < 2.5 45 – 65% of the objects seem to have two or more main components. There is not much difference between red and blue galaxies as for the fraction of objects with multiple components. However, there is a considerable difference when only red objects with strong UV bump are considered. For these galaxies, the fraction of objects with multiple components rises to 70 – 80%. This suggests that galaxies with extinction curves exhibiting a significant UV bump at 2 < z < 2.5 are either systems with many, large star-formation complexes or merging systems. This could point to the existence of an intrinsic structural difference with respect to analogous galaxies at 1<z <1.5, which appear as smooth, disc-like systems in the rest-frame UV/U-band.